1. (Field of the Invention)
This invention relates to a process for producing a carbon fiber reinforced carbon composite, in which an aggregate such as a bundle of a carbon fiber is immersed in a liquid carbonizable material, and carbonized under inert atmosphere, and if necessary, the carbonized materials are graphitized. More particularly it relates to a process for producing a high density, high strength carbon-carbon composite superior in strength, heat resistance, chemical resistance, abrasion resistance, etc. and containing almost no crack, detachment, etc.
The carbon-carbon composites produced according to the process of the present invention are useful in parts of machines, parts of electric and electronic instruments, etc.
2. (Prior Art)
It has been known that high density, high strength carbon-carbon composites can be obtained by impregnating a high strength, high modulus carbon fiber, as a reinforcement material, with a liquid carbonizable material while they are being wound into a desired shapes, or by impregnating a structure having a high strength, high modulus carbon fiber, as its principal material, e.g. woven fabrics, three dimensional woven fabrics, non-woven fabrics, unidirectionally arranged sheets, etc. with a liquid carbonizable material, carbonizing a precursor of carbon namely the liquid carbonizable material used as a matrix, in an inert atmosphere, and if necessary, further graphitizing the carbonized material.
The problems of this process are insufficiency of adhesivity between the reinforcement material and matrix carbon of the produced composite at the boundary surface thereof and generation of many cracks and detachment and reduction of mechanical strength of the composite material caused by the above-mentioned defects. Various attempts have been made to increase the adhesivity, but since there are such great temperature changes during the process of carbonization that are unbelievable in case of production of other composite materials, and since it is considered that extremely anisotropic property of crystal of carbon molecule is attributed to substantial difference in dimensional changes between the reinforcement material and the matrix, a considerably high interfacial adhesivity is required.
In order to solve the above-mentioned problems, high density, high strength carbon-carbon composite materials have been made by reimpregnating a carbonized composite material with a liquid carbonizable material and subsequently subjecting the reimpregnated material to carbonization, and repeating impregnation and carbonization steps 5 to 6 times or more to fill cracks and detached parts to attain reinforcement.
This process is a good process for producing high density high strength carbon-carbon composite materials having excellent characteristic properties, but has a drawback in that production process takes long and production cost is high because of long residence time of the carbonization step which requires a large energy consumption.
Particularly, in economical viewpoint, in case of the use of pitch as a precursor of matrix carbon, it is known that carbonization is carried out under a higher pressure in order to increase carbon yield and suppression of generation of large pores. Thus, effective impregnation and carbonization and curtailment of processing time can be possible. However, as known from the observation with a polarizing microscope, carbonized product obtained from pitch in the early stage of carbonization is a product of coarse texture having a flow pattern. It is apt to produce cracks at the time of carbonization and its strength is weak.
Even when such a shaped article is subjected to carbonization and graphitization treatment at a further higher temperature, it is difficult to expect improvement in quality of resulting products. Also, resulting products is not satisfactory as industrial material. Thus, a secondary reinforcement treatment of repetition of impregnation and carbonization becomes necessary. Thus, there is also a problem in that cost of product is high compared with the raw material cost.
Various processes have been known for reinforcing interfacial adhesivity between a reinforcement material and a matrix in carbon-carbon composite materials, but most of them are those in which surfaces of a carbon fiber are modified with a treating agent. However, a burnt modifying component introduces impurities in a carbon-carbon composite material, and as the result, there is also a problem that reduction of other qualities , e.g., reduction of chemical resistance and heat resistance occurs frequently.
In order to reinforce the interfacial adhesivity between a reinforcement material and a matrix, use of a material which is the same with the raw material of the carbon fiber of the reinforcement material, for a liquid carbonizable material as a precursor of matrix carbon is disclosed in Japanese laid open (Kokai) patent application No. 52912 of 1977. However, there is a problem that not a few cases are practically ineffective, because orientation and crystallizability of carbon molecule varies considerably according to the condition of carbonization process, and because volume change of a liquid carbonizable material at the time of carbonization is remarkably greater compared with the volume change due to the progress of crystallization of the carbon fiber.
In order to solve the above-mentioned problems, Japanese laid open (Kokai) patent application No. 209883 of 1982 discloses mixing of carbonaceous fine particles or fine particles of solid carbonaceous material into a liquid carbonizable material, as a precursor of matrix carbon. A problem of this process is difficulty of increasing mixing ratio of fine particles, because the viscosity of the liquid carbonizable material is notably increased by incorporation of fine particles, and it is difficult for the liquid carbonizable material to impregnate into an aggregate of the carbon fiber of the reinforcement material.
In Japanese laid open (Kokai) patent application No. 161144 of 1985, there is disclosed a process in which a mixed material of graphite fine powder and a carbon fiber is impregnated with a thermosetting resin, and resulting product is laminated, pressure-molded, cured and then carbonized. The problem of this process is difficulty of mixing of graphite fine powder and a carbon fiber and difficulty of shaping except those in which the carbon fiber is oriented unidirectionally. This problem can be solved by preliminarily mixing and dispersing graphite fine powder with a thermosetting resin, but deterioration of electrical properties is notable.
It is an object of the present invention to overcome the problem of loss of mechanical strength of composite material due to formation of a large number of cracks and detachment caused by insufficiency of adhesivity on the interfacial surface of a reinforcement material and a matrix at the time of shaping and carbonization of carboncarbon composite materials, and the problem of increase of production cost inevitably brought about by the necessity of removing cracks and detachment by impregnating the precursor of matrix carbon a number of times.
The above-mentioned object can be attained according to a process of the present invention.